There have been various prior approaches suggested and/or implemented for measuring nuclear magnetic resonance ("NMR") properties of earth formations surrounding a borehole to obtain evidence of the substances present.
It is well recognized that any particles of a formation having non-zero magnetic spin, for example protons, have a tendency to align with a magnetic field imposed on the formation. Such a magnetic field may be naturally generated, as is the case for the earth's magnetic field, B.sub.E. When a second magnetic field B.sub.1, transverse to B.sub.E, is imposed on the protons by a logging tool electromagnet, the protons will align with the vector sum of B.sub.E and B.sub.1 after a sufficient polarization time has passed. If the polarizing field B.sub.1 is then switched off, the protons will tend to precess about the B.sub.E vector with a characteristic Larmor frequency .omega..sub.L which depends on the strength of the earth's field B.sub.E and the gyromagnetic constant of the particle. Hydrogen nuclei precessing about a magnetic field B.sub.E of 0.5 gauss have a characteristic frequency of approximately 2 kHz. If a population of hydrogen nuclei were made to precess in phase, the combined magnetic fields of the protons can generate a detectable oscillating voltage in a receiver coil. Hydrogen nuclei (protons) of water and hydrocarbons occurring in rock pores produce NMR signals distinct from signals induced in other rock constituents.
A further NMR approach employs a locally generated static magnetic field, B.sub.0, which may be produced by one or more permanent magnets. Nuclear spins align with the applied field B.sub.0 with a time constant of T.sub.1. The angle between the nuclear magnetization and the applied field can be changed by applying an RF magnetic field B.sub.1 perpendicular to the static field B.sub.0. The frequency of the RF field must be (4.258 kHz/Gauss).multidot.B.sub.0. The angle of nutation (tilt) obtained between the nuclear magnetization and the static field is proportional to the product of B.sub.1 and the duration of the RF pulse. At the end of the RF pulse, the nuclear spins precess around the static field B.sub.0 at the Larmor frequency (4.258 kHz/Gauss).multidot.B.sub.0. The rotating component of the nuclear magnetization decays with a time constant T.sub.2 which is less than T.sub.1. Various measurements, known in the art, can be made to determine parameters of these phenomena, from which earth formation characteristics can be inferred.
For the type of operation just described, it is desirable to have the RF field, B.sub.1, perpendicular to the static field, B.sub.0, to have the static field, B.sub.0, as large as possible, and to have a static field intensity variation, as a function of position, be as small as possible in the measurement region so that a larger "resonant volume" will contribute to the measurements.
One prior art approach is described, for example, in U.S. Pat. No. 5,055,788, which discloses a nuclear magnetic resonance logging device having permanent magnets and an RF trough antenna mounted in a pad or skid that contacts the borehole wall. Measurements are made on the side of the borehole wall that the pad or skid faces. Relatively powerful rare-earth magnets can be used, and are arranged to obtain a static and substantially homogeneous magnetic field in a given volume of the formation directed to one side of the body. The trough antenna that generates the RF field is electromagnetically shielded and is directed toward the given volume of formation.
Another approach, described, for example, in U.S. Pat. No. 4,710,713, uses one or more cylindrically arranged permanent magnets in a centralized tool with a generally circumferential region of investigation around the borehole. An RF coil is wound around the outside of the magnets, and produces an RF field that is indicated as being perpendicular to the static field produced by the permanent magnets. A limitation of this centralized approach is that the RF magnetic field produced by the coil needs to pass through the magnet material, and the '713 Patent indicates that it is essential that the magnet material be non-conductive, such as a ferrite.
It is among the objects of the present invention to provide a nuclear magnetic resonance measuring apparatus that has a generally circumferential region of investigation, and overcomes limitations of prior art approaches.